Spinel synthesis

We have now presented all of the possible diffusion reactions in spinel synthesis. These are summarized as follows ... 

We have presented spinel synthesis because these systems were studied first and can be understood in a simple manner. Various silicate systems have also been studied and it has been determined that they are more representative of the general case involving solid state synthesis reactions than spinel. Let us examine the following simple silicate reaction ... 

Ganesh I (2013) A review on magnesium aluminate (MgAl204) spinel synthesis, processing and applications, hit Mater Rev 58 63—112... 

Liu, C., Zou, B.S., Rondinone, A.J. and Zhang, Z.J. (2000) Reverse micelle synthesis and characterization of superparamagnetic MnFe204 spinel ferrite nanocrystallites. Journal of Physical Chemistry B, 104 (6), 1141-1145. 

Vestal, C.R. and Zhang, Z.J. (2003) Synthesis and magnetic characterization of Mn and Co spinel ferrite—silica nanopartides with tunable magnetic core. Nano Letters, 3 (12), 1739-1743. 

Soled S.L., Iglesia E., and Fiato R.A. 1992. Copper-promoted cobalt manganese spinel catalyst and method for making the catalyst for Fischer-Tropsch synthesis. U.S. Patent 5162284. 

Figure 11. Li MAS NMR spectra of the spinel LiMn204 (structure shown on the left) as a function of synthesis temperature (adapted from ref 44). The cluster of Mn + ions formed as a result of Li substitution on the octahedral site is shown, and the resonances due to Li ions near this cluster are assigned.

Zahir, MH Katayama, S Awano, M. Synthesis and de-NOx properties of ZnO-Ga20s-AI2O3 spinel, Afofer. Chem. Phys., 2004, Volume 86, Issue 1, 99-104. 

The ample diversity of properties that the spinel compounds exhibit is derived from the fact that the possibility of synthesis of multi-component spinels by partial substitution of cations in position A and B giving rise to compounds of formula (AxA i x)(ByB 2 y)04. This accounts for the variety of reactions in which they have been used as catalyst. Moreover, partial substitution of A and B ions giving rise to complex oxides is possible, while maintaining the spinel structure. 

Needle like nanoparticles of Mg-Al mixed spinel catalysts synthesized under hydrothermal conditions were used for the synthesis of 1-methylimidazole by the gas phase imidazole methylation with methanol performed at atmospheric pressure [113]. High yield and selectivity to desired product were obtained at a temperature range between 320 and 350°C. It was proposed that one of the nitrogen atoms participates in the bonding of the imidazole with the basic site of the catalyst, and the second nitrogen atom is accessible for the reaction with electrophilic methyl species formed from methanol on acid site of the catalysts. 

Recent advances in the convenient synthesis of spinels has witnessed its application for a wide variety of catalytic reactions, such as oxidation, dehydrogenation, acylation, alkylation... 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

Studies in these laboratories have resulted in the synthesis and catalytic evaluations on a wide range of perovskites and ion modified homogeneous solid solutions for Fischer-Tropsch catalysis, copper modified spinels for higher alcohol synthesis, ion substituted perovskites for methane activation, alkali modified metal... 

Since the spinel phase must be prepared at low temperatures (by hydrothermal synthesis or by careful oxidation of magnetite at a temperature T < 300 C, for example) it has been widely suspected that some incorporation of hydrogen is needed to stabilize it. However, Schrader and Buttner have shown that pure y-Fe203 does exist, and Coey et al. have been able to prepare Fe3 j04 in the compositional range 0 < x < 0.08 by quenching non-stoichiometric magnetite prepared at 1450 °C. There is no evidence that hydrogen is needed to stabilize the system. 

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